Embodiments of the present invention relate generally to diagnostic imaging and, more particularly, to a method and apparatus capable of correcting motion errors in imaging data acquired from an object prone to motion. This motion correction is advantageously achieved by using mechanical motion data acquired from the object with one modality coincident with imaging data acquisition from the object with another and different modality.
Various imaging modalities are often used to image objects in or prone to motion, such as the heart in cardiac studies. For example, in cardiac computed tomography (CT), magnetic resonance imaging (MRI) and other imaging modalities directed to the acquisition of data from an object prone to motion, one or more motion correction techniques are generally used to reduce motion-induced artifacts in the reconstructed images. In known studies, this motion correction or compensation can add significant complexity in post processing of the images.
In one specific example, CT imaging requires more than 180 degrees of projections to formulate an image. Because of various limitations in conventional CT scanners, the time necessary to collect a complete set of projections is significant relative to object motion. For example, cardiac CT imaging is typically performed with the aid of an electrocardiogram (EKG) signal which is used to synchronize data acquisition and image reconstruction with the phase of cardiac motion. The EKG signal collected from the patient represents the electrical properties of the heart and is helpful in identifying the quiescent period of cardiac activity, which is preferred for data acquisition. Moreover, the EKG signal assists in identifying this quiescent period over several cardiac cycles. By synchronizing data collection with the quiescent period of the cardiac cycle, image artifacts and spatial resolution due to heart motion are reduced. Additionally, by consistently identifying this quiescent period in successive cardiac cycles, inconsistency between images acquired at different cardiac cycles is reduced. EKG signals can be used similarly in MR and other imaging modalities.
Although this EKG gating performs satisfactorily in most cases, there is room for improvement. Specifically, conventional EKG gating does not provide mechanical motion detection. While an EKG signal can indicate that motion is occurring or is about to occur, it cannot provide accurate real-time placement data of the heart. This is primarily a function of EKG's measuring the electrical activity of the heart and inferring mechanical motion from this electrical activity. As it is the actual mechanical motion of the heart that contributes to sub-optimal image quality, cardiac images that depend on EKG signals either require significant post processing to correct for motion artifacts or require a very high slice acquisition rate.
That is, CT reconstruction does not have a priori information on heart motion. In conventional EKG gated cardiac CT studies, the heart is presumed to be a stationary object during most of the short acquisition period identified as the quiescent period in the acquired EKG signal. Conventionally, half-scan weighting is used to suppress the impact of motion; however, its effectiveness is less than optimal since half-scan weighting reduces the contribution of CT data acquired at both ends near the 180 degree projection angular range. The amount of data to be suppressed at both ends of the dataset remains constant and therefore does not change based on each data acquisition, since there is no a priori information available. However, the amount of data to be suppressed should change based on the motion characteristics of the scanned object. For data collected roughly in the center of the 180 degree projection angular range, the data is treated in an identical manner without any weighting. Further, even with a gantry speed of 0.3 s/rotation, the central region of the projection range constitutes a 150 ms temporal window, which is prohibitively slow to completely “freeze” cardiac motion. The data acquisition window for CT systems having dual tube-detector assemblies is still between 70-80 ms which is not sufficient to eliminate heart motion. It is generally recognized that 10-15 ms temporal resolution is necessary to acquire a motion-free dataset.
It would therefore be desirable to design an apparatus and method of acquiring mechanical motion data, rather than inferring mechanical motion data, for physiologically gating CT and other image modality acquisitions to acquire motion-free datasets. It would also be desirable to incorporate the mechanical motion data into the image reconstruction process to compensate for the motion. It would also be desirable to use elasticity and other information obtained from ultrasound to map to the CT images to provide additional functional information, such as the viability of tissue. It would also be desirable to use ultrasound information to assist in identifying calcium in a CT scan and conversely use the CT anatomical information gathered in a CT scan to correct for noise in an ultrasound image. It would also be desirable to use the ultrasound tissue Doppler mode to acquire the velocity map to characterize the wall motion and map it to the CT images to provide functional information.